Saw that one on ebay. You got it at a good price, especially looking at the condition of the bodywork and the extras. You're probably going to break it instead of fixing it though?
I've personally only ever seen one cento with the leather interior. It was on the forecourt of a local garage and I stopped to have a look. This one had rear headrests as well. Managed to get a quick picture.
I asked Todger if he'd ever seen one with a factory spec leather interior, we thought it was a euorpean extra, probably very pricey for the cost of the car.
Quick google image search shows this car:
http://ipocars.com/vinfo/fiat/seicento_sport__with_leather_-2000.html
Buttflap also linked a set of front seats for sale on ebay at the moment.
https://www.fiatforum.com/auction-watch/419166-optional-leather-seats-black.html
Looking at the shape of the front headrests in your car and lack of rear headrests, they'd be from an SPI sei and the grey/black ones in mine were on an MPI sei, hence the rear headrests and the shape of the fronts.
So has the previous owner of your car fitted the interior himself, doubt you'd spec a red interior in a yellow car. Also looking at the other SPI car i linked above, you can see it doesnt have any rear headrests and the fronts are SPI as well.
But then why would it have the abarth logos embossed on the headrests? hmmm